Video and audio recorders store the information on magnetic tape which is stored in a cassette having two adjacent winding reels. The cassette is inserted into the playback device in such a way that the winding reels are actuated by gravity to lock in with suitable gudgeons of the drive mechanism of the playback device. With off-the-shelf video recorders having a rotating head cylinder it is necessary to draw the magnetic tape out of the cassette by means of a `threading` mechanism and, with the help of guides, to place the tape against the perimeter of the head cylinder. This threading process is performed by means of a threading motor which places the magnetic tape into its playing position, for example, by means of threading carriages driven by means of a cam disc.
A capstan motor drives the winding reel through a tumbler gear which is coupled to a take-off reel. In the playback mode of operation the required tape tension is maintained at a feeding reel by a friction brake which is designed as a brake band. In order to maintain uniform tape tension throughout the entire length of the tape, the actual tape tension is monitored by a tape tension sensor and evaluated to adjust the braking effect on the feeding reel dependent upon the position of the sensor.
With existing playback/record devices the brake band must be released during the search mode when tape motion is reversed because the brake band would tighten itself at the take-up reel. The release of the brake band typically is accomplished by lifting the tape tension sensor off of the magnetic tape. The existing adjusted tape tension is then lost and a loop develops in the tape. This loop must be taken up when the magnetic tape is started. Accordingly, with the existing playback/record devices it is difficult to feed a single frame for a still picture display when the direction of the magnetic tape is changed from the forward direction play mode to the backward direction search mode.
In the backward search mode the mechanical brake band has no effect and a definite given value of the tape tension needed for good tape-head contact must be obtained by the slip coupling. However, because the tape winding diameters change, an increase in the tape tension by a factor of as much as four can occur over the full length of the tape. Thus, periods of excessive tension will over stretch the magnetic tape and result in increased wear.